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Direct Photons. Mehr licht!. John Womersley Fermilab CTEQ Summer School, Madison June 2002. Hadron-hadron collisions. Photon, W, Z etc. Complicated by parton distributions — a hadron collider is really a broad-band quark and gluon collider

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direct photons

Direct Photons

Mehr licht!

John Womersley


CTEQ Summer School, Madison

June 2002

hadron hadron collisions
Hadron-hadron collisions

Photon, W, Z etc.

  • Complicated by
    • parton distributions — a hadron collider is really a broad-band quark and gluon collider
    • both the initial and final states can be colored and can radiate gluons
    • underlying event from proton remnants





Hard scattering







motivation for photon measurements
Motivation for photon measurements
  • As long as 20 years ago, direct photon measurements were promoted as a way to:
    • Avoid all the systematics associated with jet identification and measurement
      • photons are simple, well measured EM objects
      • emerge directly from the hard scattering without fragmentation
    • Hoped-for sensitivity to the gluon content of the nucleon
      • “QCD Compton process”

in the meantime
In the meantime . . .
  • Jet measurements have become much better understood
  • Lower photon cross sections and ease of triggering on EM objects lead to photon data being at much lower ET than typical jet measurements
    • Turn out to be susceptible to QCD effects at the few GeV level that
  • Photons have not been a simple test of QCD and have not given input to parton distributions, and they continue to challenge our ability to calculate within QCD
photon signatures of new physics
Photon Signatures of New Physics
  • Important to understand QCD of photon production in order to reliably search for
    • Higgs
      • H   is a discovery channel at LHC
    • Gauge mediated SUSY breaking
      • 0   G, photon + MET signatures
    • Technicolor
      • Photon + dijet signatures
      • Diphoton resonances
    • Extra dimensions
      • Enhancement ofpp   at high masses (virtual gravitons)
photon identification
Photon identification
  • Essentially every jet contains one or more 0 mesons which decay to photons
    • therefore the truly inclusive photon cross section would be huge
    • we are really interested in direct (prompt) photons (from the hard scattering)
    • but what we usually have to settle for is isolated photons (a reasonable approximation)
      • isolation: require less than e.g. 2 GeV within e.g. R = 0.4 cone
  • This rejects most of the jet background, but leaves those (very rare) cases where a single 0 or  meson carries most of the jet’s energy
  • This happens perhaps 10–3 of the time, but since the jet cross section is 103 times larger than the isolated photon cross section, we are still left with a signal to background of order 1:1.
event topology



Event topology
  • Simplest process: pp   + jet
    • Photon and jet are back-to-back in  and balance in ET
  • Experimentally we find that at about one third of the photon events have a second jet of significant ET
    • Higher order QCD processes

Back to backin parton-partoncenter of mass

boosted into lab frame

  • The greatest engineering challenge in hadron collider physics
  • To access rare processes, we must collide the beams at luminosities such that there is a hard collision every bunch crossing
    • 396 ns in Run 2 = 2.5 MHz
  • We cannot write to tape (or hope to process offline) more than about 50 events per second
    • Trigger rejection of 50,000 required
      • in real time
      • with minimal deadtime
      • and high efficiency for physics of interest
photon triggers
Photon Triggers
  • Example of how this works in DØ:
  • Level 1 (hardware trigger)
    • Requires ET > threshold in one trigger tower of the EM calorimeter (   = 0.2  0.2)
    • Total accept rate ~ 10 khZ; can allow ~ 1 kHz for electron and photon triggers
  • Level 2 (Alpha CPU, processing the trigger tower information)
    • Requires EM fraction cut and isolation cuts
    • Rejection ~ 10
  • Level 3 (Linux farm, processing the full event readout)
    • Clusters    = 0.1  0.1 cells with better resolution
    • Applies shower shape and isolation cuts
    • Rejection ~ 20
thresholds and prescales
Thresholds and prescales
  • Relatively high cross section processes like photons, with steeply falling cross sections, will be accumulated using a variety of thresholds with different prescales
  • A very simple example:
    • EM cluster > 5 GeV accept 1 in 1000
    • EM cluster > 10 GeV accept 1 in 50
    • EM cluster > 30 GeV accept all
  • Then “paste” the cross section together offline:

 1000

 50


 1

# events









signal and background
Signal and Background
  • Photon candidates: isolated electromagnetic showers in the calorimeter, with no charged tracks pointed at them
    • what fraction of these are true photons?
  • Signal
  • Background
  • Experimental techniques in Run 1
  • DØ measured longitudinal shower development at start of shower
  • CDF measured transverse profile at start of shower (preshower detector) and at shower maximum




Shower maximum


photon purity estimators

Photon purity estimators
  • Each ET bin fitted as sum of:
  • = photons
  • = background w/o tracks
  • = background w/ tracks
angular distributions
Angular distributions
  • The dominant process producing photons
  • Should be quite different from dijet production:

Can we

test this?

transformation to photon jet system



Transformation to photon-jet system

Central calorimeter coverage

BOOST of CM relative to lab


Lab pseudorapidity of jet

cos * = tanh *

* = CM pseudorapidity


Lab pseudorapidity of photon


Want uniform coverage in CM variables while respecting physical limits on detector  coverage and trigger pT

cos * = tanh *

Lines of minimum and maximum p*

p* = pT cosh *

Photon pT

min pT from trigger

 min p*

Use multiple regions tomaximize statistics;

paste distribution together using overlapping coverage

CM pseudorapidity *

photons as a probe of quark charge
Photons as a probe of quark charge
  • Inclusive heavy flavor production “sees” the quark color charge:
  • While photons “see” the electric charge:

Charm (+2/3) should be enhanced relative to bottom (-1/3)

cdf photon heavy flavor
CDF photon + heavy flavor
  • Use muon decays; pT of muon relative to jet allows b and c separation

Charm/bottom = 2.4  1.2

Cf. 2.9 (PYTHIA)

3.2 (NLO QCD)

Control sample using same dataset
    • identify 0 (= jet) instead of photon: gg  QQ events

Charm/bottom ~ 0.4

an idea for the future
An idea for the future
  • Use tt events to measure the electric charge of the top quark
    • How do we know it’s not 4/3?
      • Baur et al., hep-ph/0106341
photon cross sections at 1 8 tev
DØ, PRL 84 (2000) 2786

CDF, submitted to Phys. Rev. D

Photon cross sections at 1.8 TeV

QCD prediction is NLO by Owens et al.

data theory theory
DØ, PRL 84 (2000) 2786

CDF, submitted to Phys. Rev. D

(data – theory) / theory

±12% normalization

statistical errors only

QCD prediction is NLO by Owens et al., CTEQ4M

What’s going on at low ET?

k t smearing
“kT smearing”
  • Gaussian smearing of the transverse momenta by a few GeV can model the rise of cross section at low ET (hep-ph/9808467)

Account for soft gluon emission

CDF data  1.25

PYTHIA style parton shower

(Baer and Reno)

3 GeV of Gaussian smearing

why would you need to do this

Why would you need to do this?
  • NLO calculation puts in at most one extra gluon emission

10 GeV  2.6 GeV “kT”

50 GeV  5 GeV “kT”

In PYTHIA, find that additional gluonsadd an extra 2.5–5 GeV of pT to the system

fixed target photon production
Fixed target photon production
  • Even larger deviations from QCD observed in fixed target (E706)
  • again, Gaussian smearing (~1.2 GeV here) can account for the data
photons at hera

ZEUS 96-97

Photons at HERA
  • ZEUS data agrees well with NLO QCD
    • no need for kT ?

Have to include this “resolved” component

a consistent picture of k t
A consistent picture of kT
  • W = invariant mass of photon + jet final state
is this the only explanation
Is this the only explanation?
  • Not necessarily . . .Vogelsang et al. have investigated “tweaking” the renormalization, factorization and fragmentation scales separately, and can generate shape differences
  • This is not theoretically particularly attractive
contrary viewpoints
Aurenche et al., hep-ph/9811382: NLO QCD (sans kT) can fit all the data with the sole exception of E706“It does not appear very instructive to hide this problem by introducing an extra parameter fitted to the data at each energy”


Contrary viewpoints


isolated 0 cross sections
Isolated 0 cross sections
  • Proponents of kT point out that 0 measurements back up the kT hypothesis (plots from Marek Zielinski)
    • WA70 0 data require kT to agree with QCD (unlike WA70 photons)
    • /0 ratio in E706 agrees with theory, in WA70 does not
  • Aurenche et al. claim the opposite (hep-ph/9910352)
    • all 0 data below 40 GeV compatible, unlike photon data (E706)
    • “seems to indicate that the systematic errors on prompt-photon production are probably underestimated”
  • Predictive power of Gaussian smearing is small
    • e.g. what happens at LHC? At forward rapidities?
  • The “right way” to do this should be resummation of soft gluons
    • this works nicely for W/Z pT, at the cost of introducing parameters

Laenen, Sterman, Vogelsang,


Catani et al. hep-ph/9903436

Threshold + recoil


looks promising



Fixed Order

Threshold resummation: did

not model E706 data very well

fink and owens resummed calculations
Fink and Owens resummed calculations
  • hep-ph/0105276

DØ data

E 706 data

Agreement with data is pretty good

Does require 2 or 4 non-perturbative parameters to be set

photons at s 630 gev
Photons at s = 630 GeV
  • At the end of Run 1, CDF and DØ both took data at lower CM energy
  • Central region data are qualitatively in agreement and show akT-like excess at low ET


But . . .
  • When the UA2 data (also at 630 GeV) is added, it reinforces the impression of a deficit at large xT

What’s happening here?

Can I really ignore the datanormalization in making allthese comparisons with kT?

is it just the pdf
Is it just the PDF?
  • New PDF’s from Walter Giele can describe the observed photon cross section at the Tevatron without any kT, and predict the “deficit”

CDF (central)

DØ (forward)

Blue = Giele/Keller sets

Green = MRS99 set

Orange = CTEQ5M and L

Not all of Walter’s PDF setshave this feature: it depends on what data are input

anything similar in other final states
Anything similar in other final states?
  • b cross section at CDF and at DØ
  • Data continue to lie ~ 2  central band of theory




Cross section vs. |y|

pT > 5 GeV/c

pT > 8 GeV/c


d b jet cross section at higher p t
DØ b-jet cross section at higher pT

Differential cross section Integrated pT > pTmin


from varying the scale from 2μO toμO/2, where μO = (pT2 + mb2)1/2



DØ b-jets (using highest QCD prediction)

CDF photons  1.33

DØ photons



Data – Theory/Theory


- 0.5

Photon or b-jet pT (GeV/c)

b-jet and photon production compared

diphoton production
Diphoton production
  • Rate is very small: few hundred events in Run I (pT > 12 GeV)
  • But interesting because
    • final state kinematics can be completely reconstructed (mass, pT and opening angle of  system)
    • background to H   at LHC
  • NLO calculations available
d diphoton measurements
DØ diphoton measurements



  • Find that we need NLO QCD to model the data at large pT (small ), but NLO calculation is divergent at pT = 0 ( = )
  • Need a resummation approach (RESBOS) or showering Monte Carlo (PYTHIA) or ad hoc few-GeV kT smearing

pT ~ 3 GeV

latest nlo diphoton calculation
Latest NLO diphoton calculation
  • Binoth, Guillet, Pilon and Werlen, hep-ph/0012191

Shoulder at 30 GeV in calculation is a real NLO effect (contribution opens up with both photons on same side of the event)

photons final remarks

Photons: final remarks
  • For many years it was hoped that direct photon production could be used to pin down the gluon distribution through the dominant process:
  • Theorist’s viewpoint (Giele):

“... discrepancies between data and theory for a wide range of experiments have cast a dark spell on this once promising cross section … now drowning in a swamp of non-perturbative fixes”

  • Experimenter’s viewpoint: it is an interesting puzzle, and we like solving interesting puzzles
    • data  NLO QCD
    • kT remains a controversial topic
    • experiments may not all be consistent
    • resummation looks quite good, but how predictive is it?
    • what is the role of the PDF’s?
run 2 missing e t di em candidate
Run 2 Missing ET + di-em Candidate

+MET is a signature of gauge-mediated SUSY-breaking